1. Field of the Invention
The present invention relates to a network coding method and system, and more particular, to a network coding method and system that may differentiate the reliability of data reception and an amount of received data for each single packet with respect to each of receivers having a different channel state, in order to solve a bottleneck phenomenon of packet relay when data packets are transmitted and received between a plurality of transmitters and a plurality of receivers in a wireless communication environment.
2. Description of the Related Art
A network coding technology may enhance communication efficiency by preventing a bottleneck phenomenon in a wireless multi-hop network where a plurality of communication equipments is wirelessly connected to each other. In network coding, packet encoding may be performed by relay nodes that function to transfer a packet in a situation where a pair of nodes mutually transmit and receive data.
FIG. 1A is a diagram for describing a case where network coding is not used in a wireless multi-hop network where two nodes transmit and receive data via a single relay node according to a related art, and FIG. 1B is a diagram for describing a case where the network coding scheme is used in the wireless multi-hop network where two nodes transmit and receive data via the single relay node according to the related art.
Hereinafter, the case where the network coding scheme is used in the wireless multi-hop network where three wireless nodes, for example, a node A, a node R, and a node B, are arranged in a line will be compared with the case where the network coding scheme is not used with reference to FIGS. 1A and 1B. As shown in FIG. 1A, when the network coding scheme is not used, the node A transmits a packet 1 to the node B via the node R. The node B also transmits a packet 2 to the node A via the node R. A total of four times of packet transmissions may be required to transmit two packets 1 and 2, that is, so that the node A may transmit the packet 1 and the node B may transmit the packet 2 using the scheme of FIG. 1A.
As shown in FIG. 1B, when the network coding scheme is used, the nodes A and B may store the packets 1 and 2 transmitted by the nodes A and B, respectively, without discarding them. Instead of transmitting the packets 1 and 2 received from the nodes A and B as is, the node R may perform an exclusive OR (XOR) of the received packets 1 and 2 to encode the packets 1 and 2 into a single packet 1⊕2 and then transmit the encoded packet 1⊕2 to the nodes A and B at a one time. The node A may perform decoding by performing an XOR operation of the received encoded packet 1⊕2 and the packet 1 transmitted by the node A, that is, (1⊕2)⊕1, and thereby may extract the packet 2 that the node A desires to receive. Similarly, the node B may decode the packet 1 from the encoded packet 1⊕2 using the packet 2 transmitted by the node B. As described above, compared to the case where the network coding scheme is not used, when the network coding scheme is used, a total number of packet transmissions may be reduced from four times to three times.
In a digital communication, a process of transforming, to an electrical signal, bits of a packet to be transmitted by a transmitter corresponds to modulation. A process of decoding a received signal to a bit corresponds to demodulation. In a wireless communication environment, a channel state may be diversified based on a distance between communicators. Accordingly, when the channel state is excellent, it is possible to enhance a physical transmission rate by employing a high modulation scheme using a large number of transmission bits per symbol. On the other hand, when the channel state is poor and thus a signal-to-noise ratio (SNR) is low, it is possible to decrease a transmission rate, and to enhance data reliability by employing a low modulation scheme using a small number of transmission bits per symbol. Generally, a binary phase shift keying (BPSK) modulation scheme, a quadrature phase shift keying (QPSK) modulation scheme, a quadrature amplitude modulation (QAM) scheme, and the like may be applicable to widely used Wireless Local Area Network (WLAN) communication technologies such as an Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard.
FIG. 2A is a diagram illustrating a signal space to describe a BPSK operation scheme, and FIG. 2B is a diagram illustrating a signal space to describe a QPSK operation scheme.
In the case of a BPSK scheme shown in FIG. 2A, a single symbol includes two signals points S1 211 and S2 212 and thus a single bit is transmitted per symbol. In the case of a QPSK scheme shown in FIG. 2B, a single symbol includes four signal points S1, S2, S3, and S4 and thus two bits are transmitted per symbol. A modulated signal as above may be distorted due to noise and interference occurring when the modulated signal is transmitted from a transmitter via a wireless channel. A receiver may perform demodulation to predict which symbol is transmitted from the transmitter. A demodulation scheme generally uses a Maximum Likelihood (ML) scheme of recognizing a signal point nearest to a received signal as a signal transmitted from the transmitter.
For example, in the BPSK scheme shown in FIG. 2A, when the received signal is positioned on the right of a decision boundary 210, the received signal may be decoded to “zero” with the assumption that an original signal transmitted from the transmitter is S1 211. When the received signal is positioned on the left of the decision boundary 210, the received signal may be decoded to “1” with the assumption that the original signal transmitted from the transmitter is S2 212.
Similarly, in the QPSK scheme shown in FIG. 2B, a signal point nearest to the received signal may be recognized as a signal transmitted from the transmitter using the ML scheme. In this example, since four signal points S1, S2, S3, and S4 transmittable by the transmitter exist, a distance between the signal points S1, S2, S3, and S4 may become shorter. Compared to the BPSK scheme, the reliability of the received signal at the same SNR may be deteriorated. However, since two bits may be transmitted per signal, the QPSK scheme may achieve twice bandwidth efficiency compared to the BPSK scheme.
As described above, the network coding scheme may encode multiple packets in a node corresponding to the bottleneck phenomenon of packet relay, and may collectively broadcast the encoded packets to a plurality of nodes. Accordingly, it is possible to reduce a number of packets to be transmitted, whereas it is impossible to perform modulation in consideration of a channel state of each individual user. In a general wireless communication, a modulation scheme such as a QAM modulation scheme, a QPSK modulation scheme, and the like may be selectively used. However, when broadcasting an encoded packet using the network coding scheme, a plurality of users may need to receive the broadcast encoded packet and thus it is impossible to use a modulation scheme suitable for each of the users. Accordingly, the general existing network coding scheme may need to transmit the encoded packet using a lowest modulation allowed by a communication standard. In this case, it may cause some damage to receivers having a good channel state among the plurality of users.